1. Field of the Invention
The present invention relates to a combustion engine for a vehicle, comprising a brake booster and a crankcase ventilation system.
2. Description of Prior Art
During operation of a combustion engine, there is a leakage of fully or partly combusted exhaust gases, between the piston rings and the cylinder wall, to the crankcase. Together with e.g. condensate from the engine oil present in the crankcase they form crankcase gases which need to be ventilated. To prevent unfiltered crankcase gases from being discharged directly to the environment, they are ventilated back to the intake system of the engine. This principle is generally referred to as PCV, Positive Crankcase Ventilation.
FIG. 1 shows a combustion engine according to related art, which suggests that during low engine load operation, i.e. when the compressor 7 is not in operation and an underpressure prevails downstream the throttle valve 5, the crankcase gases are conducted from the crankcase 9, to the intake system 3 downstream the throttle valve 5, via a low load duct 11. The underpressure that prevails downstream the throttle valve 5 during low engine load operation secures the PCV-ventilation. During high engine load operation, i.e. when the compressor 7 is in operation and an overpressure prevails downstream the throttle valve 5, the crankcase gases can no longer be conducted through the low load duct 11, due to the raised pressure inside the intake system 3 downstream the throttle valve 5. Instead the crankcase gases are conducted from the crankcase 9, to the intake system 3 upstream the air compressor 7, via a high load duct 13. The underpressure that prevails upstream the compressor 7 during high engine load operation secures the PCV-ventilation in these situations.
However during low engine load operation, the crankcase gas flow through the low load duct 11 implies an addition of gas volumes to the intake system 3 downstream the throttle valve 5, which makes the control of the idling speed of the engine 1 more difficult. The reason is that the throttle valve 5 has to compensate for these extra gas volumes by closing the throttle valve 5 even further, but during idling speed operation of the combustion engine 1 this can be difficult to achieve, since the throttle valve 5 is already almost closed. Thus, speed variations will occur.
The braking system of a vehicle normally comprises a brake booster 27, which serves to enhance the force from the foot of the driver when he operates the brake pedal. Thus, an underpressure is created within the brake booster 27, which assists the driver when operating the brake pedal. The underpressure is commonly created by fluidly connecting the brake booster 27 with the intake system 3 downstream the throttle valve 5 by means of an evacuation duct 29. When the driver activates the brake pedal, the ambient atmosphere leaks into the brake booster 27 and is thereafter evacuated to the intake system 3 via the evacuation duct 29. Hereby, the pressure within the brake booster 27 will assume the same pressure as in the intake system 3.
To further reduce the pressure within the brake booster, a venturi tube 15 can be employed. This venturi tube 15 forms part of a by-pass duct 12 and comprises a narrow venturi tube portion 19 forming a flow resistance within the by-pass duct 12. A first end 14 of the by-pass duct 12 is fluidly connected to the intake system 3 upstream the throttle valve 5, while a second end 16 is fluidly connected to the intake system 3 downstream the throttle valve 5. During low engine load operation there is a pressure drop over the throttle valve 5, resulting in an air flow from the first end 14 to the second end 16 of the by-pass duct 12. When air passes the narrow venturi tube portion 19, the kinetic energy of the air increases while its pressure decreases. Since the pressure that prevails just downstream the venturi tube 15 is the same as the pressure within the intake system 3 downstream the throttle valve 5, the pressure within the narrow venturi tube portion 19 will decrease below the pressure within the intake system 3 downstream the throttle valve 5. Moreover, the evacuation duct 29 is fluidly connected to this narrow venturi tube portion 19, and therefore the pressure within the evacuation duct 29 and the pressure within the brake booster 27 will assume the same pressure as the pressure within the narrow venturi tube portion 19, i.e. below the pressure inside the intake system 3 downstream the throttle valve 5.
However, since the air flow through the evacuation duct 29 also implies an addition of gas volumes to the intake system downstream the throttle valve 5, the control of the idling speed of the engine 1 will be even more difficult.